Power-consulting circuits that sample, convert, readout, amplify or otherwise process data, or signals that represent data, are known in the art. For example, an image sensor typically includes a plurality of pixel sensors arranged in an array. To sample the signals generated by the pixel sensors, a column switch for each column of the array selectively couples a column line for that column to an output bus while one or more rows of the array are selected. That is, selecting one or more rows of the array causes the pixel sensors of the selected rows to be coupled to the column lines for the columns to which the pixel sensors belong, and selecting the column line for a particular column causes that column line to be coupled to the output bus, which allows the pixels of that column that are within the one or more selected rows to be sampled at the output bus.
For an image sensor whose pixel sensors are active pixel sensors, a DC source is typically associated with each column to generate and provide a bias current to the active pixel sensors of that column. More particularly, each active pixel sensor includes a follower-type pixel amplifier that provides a buffered voltage output signal. The buffered voltage output signal is representative of an integrated charge generated by a photodiode of the active pixel sensor in response to incident light. The bias current generated and provided by the DC source enables the pixel amplifier for each pixel sensor to provide the buffered output voltage to a column line for the column to which that pixel sensor belongs, as described in the previous paragraph.
Conventionally, the DC bias current source for each column remains activated (and, thus, consumes power) even when the column line for that column is not selected to be coupled to the output bus. It is desirable to minimize the amount of power consumption in such circuits (and/or for other power consuming circuits, which when not operating, are such that signals generated by photosensors of the pixel sensors are prevented from being reliably sampled or read from the pixel sensor array, or otherwise processed), particularly when the circuits are part of a portable, battery-powered device.
Conventional “power save” circuitry typically operates by putting power consuming circuitry into a sleep mode based on measuring some period of inactivity. Then some action (independent of the normal operation of the power consuming circuitry) causes the power consuming circuitry to be reactivated. An example of such an action is the activation of a user interface feature. During the reactivation period, the power consuming circuitry is non-operable (or, at least is not reliably operable) and not available for use. It is desirable, then, to control the consumption of power by such power consuming circuitry, but to do so in a way that does not interfere with the reliable operation of the power consuming circuitry and that does not require user intervention or other mechanisms for monitoring activity in the device. For example, with respect to an image sensor, it is desirable to control the consumption of power by bias current source circuitry associated with the columns, but to do so in such a way that the output values of the follower-type amplifiers for the pixels of each column are reliably available when required (e.g., when the column line for that column is selected to be coupled to the output bus). Similarly, other power consuming circuits such as column amplifiers in an image sensor may also be powered on in a manner that makes them operational only when or just before they are needed for readout of the signals.